1. Field of the Invention
The present invention relates to an electronic control unit mounted on, for example a vehicle, adapted to control various types of loads to be driven connected thereto, more particularly to an electronic control unit capable of detecting an abnormal state in which the control cannot be performed properly.
2. Description of the Related Art
A type of electronic control unit is commonly used for an electronic throttle system of a vehicle in which a throttle valve is driven by a motor. The throttle valve is disposed in an inlet system of an internal combustion engine of the vehicle and the electronic control unit is adapted to control driving the motor in the electronic throttle system.
In this electronic control unit (hereinafter referred to ECU), a microprocessor is used to calculate a target throttle position of the throttle valve in response to an operating input of an accelerator which is determined by a step amount on an accelerator pedal. The microprocessor is adapted to output a control signal responding to the target throttle position to a motor driver so as to control the motor. The motor driver is configured to drive the motor by supplying power in response to the control signal received by the microprocessor. As a result, the throttle position is maintained to be the target throttle position.
Unlike a conventional type throttle system e.g. a mechanical cable-driven throttle system in which the accelerator pedal and the throttle valve are connected by an accelerator cable, the electronic throttle system controls the throttle position of the throttle valve by using a microprocessor. Hence, in case the microprocessor becomes abnormal condition due to unexpected reasons, the microprocessor cannot control the motor correctly so that the throttle position may not be controlled properly.
In this regard, following technique is commonly used in the electronic throttle system to implement a fail safe function. For instance, Japanese Patent Application Laid-Open Publication number JP-2003-343326 and JP-2007-2760 disclose a fail safe function having a control circuit implemented, the control circuit being adapted to detect an abnormal event of the electronic throttle system where the motor (i.e., motor for controlling the throttle valve) cannot be controlled by the microprocessor properly and adapted to forcibly suspend driving the motor when the abnormal event is detected.
Specifically, as an example of an electronic throttle system having such a fail safe function, the system is provided with an ECU including a judgment circuit for judging whether or not the motor is controlled properly by the microprocessor. The judgment circuit has two input signals including a judgment data and a reset command, which are transmitted by the microprocessor via a communication interface. The judgment data consist of pass data, i.e., data indicating a state in which the motor is controlled normally by the microprocessor, and fail data, i.e., data indicating an abnormal state for controlling the motor. The judgment circuit is configured as a logic circuit and being reset when the power is supplied, i.e., power on reset or the reset command is activated.
As a fail safe function, the judgment circuit generates a lock signal to suspend driving the motor, the lock signal being active when the judgment data indicates a fail data or the reset event occurs. Further, the lock signal remains active once the fail data is received so that the motor is locked to be suspended.
Meanwhile the lock signal becomes inactive when a pass data following the reset command is received. The reset command is usually transmitted by the microprocessor only when the power on reset is released. Therefore, once the abnormal event is detected, the lock signal can remain active even when the pass data is received afterwards.
However, as described above, in the ECU conventionally used, the lock signal from the judgment circuit can be reset based on the reset command transmitted from the microprocessor. Hence, it is considered that following problems arise.
When after the lock signal becomes active due to receiving a fail data, if a fault occurs on the microprocessor thereby accidentally transmitting the reset command and transmitting a pass data afterwards, the lock signal from the judgment circuit becomes inactive. As a result, the suspended state of the motor is released, that is, the throttle valve may be driven by the motor in response to improper signals from the microprocessor.